What Causes A Red Rash At The Back Of The Leg?

i have a red rash on the bak of my leg starting from the ack of my knee which is approx 10 inches wide by 9 inches long now from what starte to be the size of less than a dime in tree days with a white itchy ring around it; in the center are 6-7 little clear painul blisters growing... the samething occured on my other leg a month ago but alf that size and it went away after a few days. this new keeps growing